Removing Ci::LegacyStage

Problem to solve

The starting point of this issue: #218536 (comment 387394549)

We need to remove Ci::LegacyStage and use Ci::Stage for replacement.

Proposal

When replacing Ci::LegacyStage with Ci::Stage there are a few places to consider:

• removing Ci::Pipeline#legacy_stages
  • replacing legacy_stages in templates
• latest_statuses_ordered_by_stage (consider import_export/all_models.yml)
• removing Ci::Pipeline#legacy_stage
  • replacing legacy_stages in Projects::PipelinesController

Additional notes from comment:

• When we remove Ci::LegacyStage we should evaluate whether it's possible to remove StageUpdateWorker and Ci::EnsureStageService. This would be an additional step towards deprecating stage related columns of ci_builds table.
  ✅ * Before we remove Ci::LegacyStage we should make Ci::Stage presentable so we can replace pipeline.legacy_stages in the view with pipeline.stages. Perhaps we need to make this behind a feature flag an in separate MR prior to actually removing Ci::LegacyStage <- Ci::Stage is already presentable: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/presenters/ci/stage_presenter.rb

Further details

• related issue: #28406 (closed)
• related issue: #213103 (closed)

Availability & Testing

• Unit test changes - Required unit tests to be updated according to changes
• Integration test changes - not required
• End-to-end test change - not required
• package-and-qa should be run in MR with these changes to ensure no regressions is introduced.